FIG. 1 illustrates an airborne localization. The transmitter is at the position (x0,y0,z0). The carrier at the instant tk is at the position (xk,yk,zk) and perceives the transmitter at the angles of incidence (θ(tk,x0,y0,z0), Δ(tk,x0,y0,z0)). The angles θ(t,x0,y0,z0) and Δ(t,x0,y0,z0) evolve in time and depend on the position of the transmitter as well as the trajectory of the carrier.
The angles θ(t,x0,y0,z0) and Δ(t,x0,y0,z0) are determined relative to a network of N antennas that can be fixed beneath the carrier as shown in FIG. 2.
There are several existing techniques to determine the position (xm, ym, zm) of a transmitter. These techniques of localization differ especially in the parameters which are estimated instantaneously at the level of the network of sensors. They can be classified as follows.
Use of Goniometry
These techniques are known and used in the prior art. In most cases, they are based on a 1D azimuthal goniometry. The azimuths θkm=θ(tk,xm,ym,zm) associated with the mth transmitter are measured for different instants tk. In using the position (xk,yk,zk) of the carrier at the corresponding instant k, a position (xmk,ymk,zmk) of the transmitter m is estimated by a ground intersection. The position (xk,yk,zk) of the carrier is given by a GPS unit, its orientation is obtained by a compass in the case of a ground carrier and by an inertial navigation system in the case of an aircraft. From all the positions (xmk,ymk,zmk), the method extracts data with which it is possible to determine the M dominant positions (xm,ym,zm) of the incident transmitters. The localization is obtained by triangulation or by ground intersection (2D goniometry). The drawback of triangulation techniques is that they require major movement. Furthermore, goniometry techniques require the use of a network of non-ambiguous sensors. This has the drawback of necessitating a calibration table and restricting the size of the sensor network and therefore giving incidence values of limited precision.
Use of the Phase Difference Between Two Remote Sensors
The inter-sensor phase difference Δφ(tk,x0,y0,z0) depends on the positions of the two sensors as well as the incidence θ(tk,x0,y0,z0), Δ(tk,x0,y0,z0)) of the transmitter. This phase, which depends on time, is directly related to the position (x0,y0,z0) of the transmitter. Consequently, studying the function of time Δφ(t,x0,y0,z0) makes it possible to deduce the position (x0,y0,z0) of the transmitter. In this group of applications, the two sensors are distant in order to augment the precision of measurement of the phase. This has the drawback of causing variations in the phase difference Δφ(t,x0,y0,z0) as a function of time over more than 2π and the technique then necessitates a step enabling the phase to be made to vary by more than 2π. Furthermore, in this technique, the phase is measured by carrying out a direct intercorrelation between two sensors, and cannot be used to deal with the multiple-sensor case.
Use of the Measurement of the Carrier Frequency of the Transmitter
These techniques make use of the fact that the estimated carrier frequency is the sum of the carrier frequency of the transmitter and the Doppler shift due to the speed of movement of the carrier. The Doppler shift has the advantage of depending on the position (x0,y0,z0) of the transmitter and of also being a function of the time Δf(t,x0,y0,z0). Consequently, studying the function of the time Δf(t,x0,y0,z0) makes it possible to deduce the position (x0,y0,z0) of the transmitter therefrom. However, the measurement of this Doppler shift has the drawback of necessitating transmitters with particular waveforms. This measurement of frequency can be done by cyclical techniques in which it is assumed that the signal sent is non-circular.
Use of Propagation Times
These techniques use the differences in propagation time between antennas (TDOA or time difference of arrival) which are directly related to the respective differences between the transmitter and the different antennas and therefore to the position (x0,y0,z0) of the transmitter. The use of at least three antennas that are sufficiently spaced out enables the position (x0,y0,z0) of the transmitter to be deduced by hyperbolic localization. The drawback of these techniques is that they cannot be implemented in a single-carrier context owing to the considerable spacing required between antennas. Furthermore, in these techniques, the time difference is measured by the direct performance of an inter-correlation between two sensors. This approach cannot be used to deal with the case involving multiple transmitters.